Device for driving a semiconductor laser by a plurality of driving elements

ABSTRACT

A semiconductor laser apparatus comprises a semiconductor laser, a photodetector, first and second variable gain units, a controllable amplifying unit a driving unit, and a compensation unit. A control signal is supplied to one of input terminals of each of the first and second variable gain units and an output signal of the photodetector is negative-fed back to the other of the input terminals of each of the first and second variable gain units. A varied gain of an error between the control signal and the output signal is output independently of each of the first and second variable gain units. The controllable amplifying unit amplifies the error signal from the first variable gain unit with desired frequency characteristics. The driving unit includes a plurality of driving elements connected in parallel and supplies a driving current to the semiconductor laser in accordance with a drive control signal from the controllable amplifying unit. The compensation unit negatively feeds back a compensation current to the negative feedback terminals to compensate a phase delay on the basis of the error signal output from the second variable gain unit.

CROSS-REFERENCE TO THE RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/465,413 filed Jun. 5, 1995 now U.S. Pat No. 5,579,329.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor laser apparatus used inan optical disk apparatus, a laser printer, an optical datacommunication system, and the like.

2. Description of the Related Art

Semiconductor lasers are widely used in systems of optical diskapparatuses, which are large capacity memory apparatuses, or laserprinters, because of advantages of easy modulation of direct lightintensity, a small size, low power consumption and high efficiency.

Conventional semiconductor lasers, however, have a disadvantage in thatthe amount of emission light varies due to the followingcharacteristics:

(1) A variation in differential quantitative due to a temperaturevariation and a variation with the passing of time,

(2) A variation in threshold current due to a temperature variation andreflection light (return light), and

(3) Occurrence of mode hopping noise due to reflection light (returnlight).

In order to drive the semiconductor laser, a control circuit formonitoring and stabilizing the amount of output light of thesemiconductor laser is indispensable. In particular, in an optical diskapparatus, in order to increase the memory capacity and data transferrate, light intensity modulation with higher precision and reduction innoise at the time of reproduction are required.

A wide band front APC method is known as a method for reducing lasernoise with a currency available semiconductor laser used as a lightsource of an optical disk apparatus (e.g., TAGUCĤI, HOSHINO:“High-Precision Laser Control System (11)” in Optical Disk Apparatuses,General Meeting in Spring 1991 of the institute of Electronics,Information and Communications Engineering, C-372, etc.).

In the wide band front APC method, a light beam actually radiated on anoptical disk in a recording/reproducing mode, i.e., part of a front beamof a semiconductor laser is guided to, and detected by, a photodetector.A detection signal from the photodetector is used for light outputcontrol of the semiconductor laser. According to this method, thecontrol band is increased and thus the laser noise is reduced.

Regarding the wide band front APC, it is important how the control bandis increased in relation to the reproduction signal band. A techniquefor making the control band of the wide band front APC wider than thereproduction signal band is disclosed in, e.g., Jpn. Pat. Appln. KOKAINo. 4-209581 (the title of the invention: “Semiconductor LaserApparatus”).

The semiconductor laser apparatus is provided with an error detectioncircuit for outputting an error signal representing an error between anoutput signal negatively fed bank from a photodetector for detecting anoutput beam of the semiconductor laser and an external control signal. Afeedback loop is formed which controls a drive current for thesemiconductor laser on the basis of the error signal. In addition, thesemiconductor laser apparatus is provided with a compensation circuitfor negatively feeding back a compensation current for compensating aphase delay of the feedback loop to an input terminal of the errordetection circuit.

In the prior art, however, no consideration has been paid to thevariation in feedback amount of the feedback loop due to theaforementioned variation in differential quantitative efficiency or thevariation with the passing of time of the semiconductor laser, or thevariance in adjustment of optical systems among apparatuses.

Nor has consideration been paid to the variation in frequencycharacteristics due to a delay in the semiconductor laser or a variationin junction capacitance in the photodetector.

If the aforementioned variation feedback amount or frequencycharacteristics occurs, laser noise cannot fully be reduced, inparticular, in an information recording/reproducing apparatus such as anoptical disk apparatus. Furthermore, unnecessary noise occurs due todegradation in transient response characteristics to noise.

Moreover, the degradation in transient pulse response characteristics atthe time of recording poses a more serious problem, since it result in arecording mark variation and greatly loses a reproduction margin.

On the other hand, in a laser printer, etc., there is a demand for amuch higher light turn/off ratio, i.e., a light turn on/off ratio.However, in a turn-off region, i.e., a threshold lower than laseroscillation, the feedback efficiency is greatly lowered and high lightturn-off ratio cannot be obtained. This problem will now be describedwith reference to simulation results shown in FIGS. 1 and 2.

FIG. 1 shows a response waveform of a monitor PD current at the time ofturn on/off. It is understood that the control system is deterioratedbecause the light is not fully turned off at the time instant of turnoff, although the light has high-speed responsiveness and stable at thetime of turn on.

FIG. 2 shows a response waveform of an LD (laser diode) driving currentat the time of turn on/off. It is understood that the driving currentcan be controlled to only the level of threshold current at the timeinstant of turn off.

As has been mentioned above, in the conventional semiconductor laserapparatus, no consideration is paid to the variation in feedback amountof the feedback loop due to the variation in differential quantitativeefficiency or the variation with the passing of time of thesemiconductor laser, or the variance in frequency characteristics due toa delay in the semiconductor laser or a variation in junctioncapacitance in the photodetector.

Thus, in the case of the information recording/reproducing apparatussuch as an optical disk apparatus, the effect of laser noise reductionis not sufficiently and unnecessary noise is caused by the degradationin transient response characteristics to noise.

In the case where the performance in light turn-off ratio is required,the feedback efficiency decreases greatly at a level lower than thelaser oscillation threshold and a high light turn-off ratio cannot beobtained.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a high-poweredsemiconductor laser apparatus wherein the feedback amount of a feedbacksystem for negatively feeding back a driving current of a semiconductorlaser is automatically compensated without damaging the dynamic range ofa controllable amplifier, the variation in characteristics due to adelay in a semiconductor laser or a variation in junction capacitance inthe photodetector, and a high turn-off ratio is obtained.

To attain the above object, there is provided a semiconductor laserapparatus comprising:

a semiconductor laser;

a photodetector for detecting an output beam of the semiconductor laser;

a driving unit having a plurality of driving elements connected inparallel, for driving the semiconductor laser;

a controllable amplifying unit for receiving an externally suppliedcontrol signal and a monitor signal of the photodetector and supplying adrive signal determined by the control signal and the monitor signal ofthe photodetector to the driving unit, thereby feedback-controlling theoutput beam of the semiconductor laser; and

a compensation unit for actively controlling a phase of a feedbackcontrol loop constituted by the photodetector, the driving unit and thecontrollable amplifying unit in accordance with temporal and electricalbehaviors of the feedback control loop.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention and, together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a graph showing a monitor PD current waveform at the time ofon/off control of a semiconductor laser in a conventional semiconductorlaser apparatus;

FIG. 2 is a graph showing an LD (laser diode) driving current waveformat the time of on/off control of the semiconductor laser in theconventional semiconductor laser apparatus;

FIG. 3 is a block diagram schematically showing a semiconductor laserapparatus according to a first principle of the present invention;

FIG. 4 is a detailed circuit diagram of a semiconductor laser apparatusaccording to a first embodiment of the present invention;

FIG. 5 is a circuit diagram showing a loop constituted by a differentialvariable amplifier and a compensation circuit shown in FIG. 4;

FIG. 6 is a graph showing a phase progress effect obtained when the gainof the differential variable amplifier shown in FIG. 4 is varied;

FIG. 7 is a detailed circuit diagram of a semiconductor laser apparatusaccording to a second embodiment of the present invention;

FIG. 8 is a detailed circuit diagram of a semiconductor laser apparatusaccording to a third embodiment of the present invention;

FIG. 9 is a block diagram showing the principle of parallel driving of asemiconductor laser;

FIG. 10 is a graph showing closed-loop frequency characteristics in theparallel driving;

FIG. 11 is a graph showing pulse response characteristics of a laserdiode of an optical system;

FIG. 12 is a block diagram of an equalizing compensation system;

FIG. 13 is a graph showing simulation results of closed-loop frequencycharacteristics in the equalizing compensation system; and

FIG. 14 is a graph showing simulation results of pulse responsecharacteristics of a laser diode of an optical system in the equalizingcompensation system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A semiconductor laser apparatus according to a first principle of thepresent invention will now be described with reference to FIG. 3.

As is shown in FIG. 3, the semiconductor laser apparatus according tothe first principle comprises a semiconductor laser 510, a photodetector520, a driving unit 530, a controllable amplifying unit 540 and acompensation unit 550. The photodetector 520 detects an output lightbeam from the semiconductor laser 510. The driving unit 530 supplies adriving current to the semiconductor laser 510. The controllableamplifying unit 540 APC-controls the output of the semiconductor laserby feedback control. For this purpose, the controllable amplifying unit540 receives a control signal from terminals 531 541 and an output lightsignal of the photodetector 520 and supplies a driving signal determinedby the control signal and output light signal to the driving unit 530.The compensation unit 550 actively controls the phase, in particular, aphase delay, of a feedback loop constituted by the photodetector 520,driving unit 530 and controllable amplifying unit 540, in accordancewith temporal and electrical behaviors of the feedback loop.

FIG. 4 shows a first embodiment of the semiconductor laser apparatusaccording to the first principle shown in FIG. 3. As is shown in FIG. 4,the semiconductor laser apparatus of the first embodiment comprises anintegrated semiconductor laser control circuit 1, a semiconductor laser23 connected to the laser control circuit 1, a photodetector 24 fordetecting an output light beam of the semiconductor laser 23 and aplurality of resistors connected externally to the semiconductor lasercontrol circuit 1.

The structure of the semiconductor laser control circuit 1 will now bedescribed. A control current signal is input to an input terminal 30from the outside. The control current signal is converted to a controlvoltage signal by a current/voltage converter 2. The control voltagesignal is supplied to each of a controllable amplifying unit 10 and avariable gain type differential amplifier s functioning as a secondvariable gain means. The controllable amplifying unit 10 comprises avariable, gain type differential amplifier 6 functioning as a firstvariable gain means, an operational amplifier 7 and a differentialamplifier s for subjecting outputs of both amplifiers 6 and 7 tosubtraction. The controllable amplifying unit 10 outputs a drivingcontrol signal.

The driving control signal output from the differential amplifier 8 isshifted to a desired potential level by a level shift 11. Thelevel-shifted driving control signal is input to each of a buffer 18, atransistor 12 for detecting a driving current of the semiconductor laser23, and a transistor 13 for supplying a driving current to thesemiconductor laser 23.

The damping capacity of the transistor 13 is about 100 times that of thetransistor 12. About 1% of the driving current flows through thetransistor 12, and the driving current is monitored on the basis of avoltage drop in an external resistor R1. The comparator 14 compares thevoltage drop with a reference voltage Ref3 and outputs a comparisonresult to the outside via a buffer 16. In addition, the comparator 14limits the driving current by lowering the output potential of the levelshift 11.

A window comparator 15 determines the control state when the outputvoltage of the operational amplifier 7 is in a range between Ref1 andRef2, and outputs a determination result to the outside via a buffer 17.

In addition, the semiconductor laser control circuit 1 includes a powersupply monitor 21 for detecting a decrease in power supply voltage andautomatically halting the control operation, and a power save circuit 22for starting and stopping the control operation by external control.

The control voltage signal from the current/voltage converter 2 is amodulated signal voltage which is intensity-modulated so as torepresent, e.g., digital information. The modulated signal voltage issupplied to an inverted input terminal of the controllable amplifyingunit 10 as a control current via a resistor element Ri. A referencevoltage generated by a current/voltage converter 3 having the samestructure as the current/voltage converter 2 is input to thenon-inverted input terminal of the controllable amplifying unit 10.

An anode of the photodetector 24 is connected to a terminal 39. Amonitor current from the photodetector 24 is negative-fed back to theinverted input terminal of the controllable amplifying unit 10 via theterminal 39. Thus, the output beam from the semiconductor laser 23 isintensity-modulated in proportion to the modulated signal voltage whichis the control voltage signal from the current/voltage converter 2, Thatis, the semiconductor laser 23 is auto-power-controlled.

The gain of the operational amplifier 7 ran be varied by an externalresistor Rz, and the zero-point frequency for avoiding the influence ofjunction capacitance of the photodetector 24 can be optimized.

Voltage/current converters 19 and 20 pull up potentials provided frominput terminals 44 and 43 in their insides on the basis of referencevoltage and resistance. The pull-up levels of the voltage/currentconverters 19 and 20 can be set by voltage input or external resistorsRg1 and Rg2. Output currents from the voltage/current converters 19 and20 are input to the variable gain type differential amplifiers 6 and 5,respectively.

In general, the dynamic range of an output from a variable gain typedifferential amplifier varies greatly due to a variation of gain. Thus,if the variable gain type differential amplifier is provided at the rearstage of the circuit (controllable amplifying unit), the dynamic rangeof the control system lowers greatly. It is therefore desirable that thevariable gain type differential amplifier be provided at the first stageof the circuit (controllable amplifying unit). That the differentialamplifier has a gain varying function is indispensable to compensate avariance among devices and optical systems.

A description will now be given of the effect of separation between thevariable gain type differential amplifier 5 and controllable amplifyingunit 10, which is one of the features of the present invention. Anoutput terminal of the variable gain type differential amplifier 5 isconnected to a terminal 33. The terminal 33 is connected to a terminal32 on the outside of the semiconductor laser control circuit 1. Thus, aphase delay compensation current, i.e., a compensation current tocompensate degradation of response characteristics due to an influenceof junction capacitance of the semiconductor laser 24, is generated by acompensation circuit 9 comprising an RC series circuit. In this case, aloop constituted by the variable gain type differential amplifier 5 andcompensation 9 can be regarded as an independent feedback loop, as shownin FIG. 5. Specifically, the phase characteristics of Vout alone, i.e.,an input to the controllable amplifying unit 10, may be considered withrespect to the operation of the semiconductor laser 23.

FIG. 6 shows the phase progress effect obtained at the time the gain ofthe variable gain type differential amplifier 5 has been varied. As isshown in FIG. 6, sufficient phase progress effect is obtained only byvarying the gain of the differential amplifier 5. There is no need toalter the constant of the compensation circuit 9. Thus, the apparatus ofthis embodiment can be built in an integrated circuit and iscost-effective.

On the other hand, a necessary gain in the control system can be setwithout adjustment, if the control gain cross frequency, i.e., controlband is determined at first. In order to achieve this effect, it isnecessary to flatten the frequency characteristics of the variable gaintype differential amplifier 5 up to 1.5 times the target control band.

As has been described above, in the present embodiment, even if the gainof the first variable gain amplifying means provided at the first stageis varied, the dynamic range of the rear-stage controllable amplifyingmeans with a higher gain is not degraded. In particular, high-precisioncontrol less vulnerable to a variance in threshold current can beachieved.

Furthermore, since the second variable gain type amplifying means isindependent of the feedback control loop, the phase compensation of thefeedback can be adjusted independently. Since the gain can be varied,the constant of the compensation means need not be altered. Thus, thecost for parts and adjustment can be reduced.

A semiconductor laser apparatus according to a second embodiment of thepresent invention will now be described with reference to FIG. 7.According to the second embodiment, when the semiconductor laserapparatus is used at a low power supply voltage, a driving current canbe amplified without degrading pulse characteristics since asemiconductor laser 22 can be driven by a plurality of drive transistorsarranged in parallel. In this parallel driving, a transient compensationfor preventing a feedback control loop from overshooting is adopted, asis a high-frequency addition for adding a high-frequency oscillationsignal to an output signal of a variable gain amplifier GCAMP1. Theparallel driving and transient compensation will be described in detailwith reference to FIGS. 9 to 11.

The semiconductor laser apparatus of the second embodiment can favorablybe incorporated into an information recording/reproduction apparatussuch as an optical disk apparatus. Since the currently-used optical diskapparatus is equipped with a monitoring photodetector having goodcharacteristics, it does not need any equivalent for the equalizingfunction of a semiconductor laser apparatus according to a thirdembodiment of the present invention, which is shown in FIG. 8 and willbe described later.

In FIG. 7, the structural elements common to those shown in FIG. 4 aredenoted by like reference numerals and their descriptions are omitted.

Referring to FIG. 7, an output signal of an input I/V converter circuit2 is inverted by an inverting buffer AMP101, and the inverted signal issupplied to a drive transistor 105 via a level shift 103. An outputsignal of the drive transistor 105 is added to that of a controllingdrive transistor 13 at a terminal (LDK) 37. The added signal is suppliedto the semiconductor laser 22. The gain of the drive transistor 105 isdetermined by a resistor (Rb) externally connected to a terminal (IB)106 (open-loop) control). An output signal of the inverting bufferAMP101 is led to a compensation circuit 102 constituted by a seriescircuit of both a resistor Rtr and a capacitor ctr. A high-frequencycompensation current output from the circuit 102, which enables thetransient compensation, is supplied to each of inverting input terminalsof the variable gain amplifiers GCAMP1 and GCAMP2.

In the semiconductor laser apparatus shown in FIG. 7, the open-loopcontrol is adopted in addition to the feedback control system shown inFIG. 4 and, in this case, a feedback current of the open-loop control,corresponding to an input signal, has to be exactly fed back to afeedback point, i.e., an inverted input terminal point of the variablegain amplifier GCAMP1, in order to prevent an influence upon thefeedback control system. If inexact feedback is performed, the feedbackcontrol system operates so as to compensate for a componentcorresponding to the inexact feedback, with the result that an overshootof output beam occurs and the characteristics are degraded. Thehigh-frequency compensation current functions to compensate a decreasein feedback current in a high-frequency band of the semiconductor laser22 and photodetector 23, prevent an overshoot from occurring, and avoidan influence upon the feedback control system.

Furthermore, in the apparatus shown in FIG. 7, a balance output signalof the variable gain amplifier GCAMP1 and those of a high-frequencyoscillator 112 having terminals 110 and 111 are added by adders 113 and114, respectively. Since the added high-frequency oscillation signalsare supplied to the semiconductor laser 22 through controllableamplifier HGAMP 7, level shift 11, and drive transistor 13, a feedbackloop is not influenced by variations in loop gain. Since, moreover,moreover, the gain of the variable gain amplifier GCAMP1 is decreased(to smaller than 6 dB), the feedback loop cannot be influenced by thevariations in loop gain. It is usually when a high gain amplifier isarranged ahead of a point where the high-frequency oscillation signalsare added that the variations in loop gain exercise an influence uponthe feedback loop. In this case, the amplifier generates undesirablecomponents to attenuate the added signals.

If the high-frequency oscillation signals are added to the output of thevariable gain amplifier GCAMP1, superimposition of high-frequencyoscillation signals, which is not influenced by variations in loop gainor does not influence the feedback loop, can be achieved.

As has been described above, in the second embodiment, two drivingelements are arranged in parallel so as to supply a driving current inresponse to an input signal Furthermore, an overshoot occurring duo toparallel driving of the two driving elements is transient-compensated bydelivering a high-frequency component of the input signal to thefeedback point.

As a result, the open-loop control is performed and the characteristicsof the semiconductor laser can be fully exhibited. Therefore, pulsecharacteristics such as a decrease in pulse rising time can be improved.

In the vicinity of a light turn-off region, the controllable amplifierHGAMP 7 comes into a saturated region and the pulse characteristicsdeteriorate greatly. According to the parallel driving, the outputamplitude of the controllable amplifier is reduced; thus, the amplifierperforms the control without coming into the saturated state, and thelight turn-off ratio is remarkably enhanced. For this reason, the loadon the controllable amplifier is reduced and the light turn-off ratio isimproved.

If the high-frequency oscillation signals are added to the output of thevariable gain amplifier GCAMP1, superimposition of high-frequencyoscillation signals, which is not influenced by variations in loop gainor does not influence the feedback loop, can be achieved.

A semiconductor laser apparatus according to a third embodiment of thepresent invention Will now be described with reference to FIG. 8. Likethe apparatus of the second embodiment, that of the third embodimentadopts parallel driving and transient compensation. It also adoptsequalizing compensation for correcting a control signal input to afeedback control loop in order to prevent a sag of pulse responsecharacteristics of an output beam of a semiconductor laser 22 fromoccurring. The equalizing compensation will be described in detail withreference to FIGS. 12 to 14.

The semiconductor laser apparatus of the third embodiment can favorablybe incorporated into an information recording apparatus such as a laserbeam printer. An equalizing function is essential to the laser beamprinter since a photodetector is incorporated into a semiconductorlaser.

In FIG. 8, the structural elements common to those shown in FIG. 7 aredenoted by like reference numerals and their descriptions are omitted.The semiconductor laser apparatus incorporates a D/A converter sectionfor converting a digital signal into an analog control signal in orderto control an output beam in accordance with external digital data so asto easily perform the gradation control. The D/A converter sectionincludes input terminals 120 to 128, a latch 129 and an A/D converter130. According to the third embodiment, the equalizing compensation isexecuted by adding an output signal of a buffer 101 in a current mode ata feedback point through an externally-arranged compensation circuit 131including a resistor Req and a capacitor Ceq. The equalizingcompensation is intended to compensate for a control signal input to afeedback control loop in order to prevent a sag of pulse responsecharacteristics of an output beam of the semiconductor laser 22 fromoccurring.

The parallel driving and transient compensation in the second and thirdembodiments will now be described in detail. The parallel drivingcontributes to an improvement in driving response characteristics of asemiconductor laser. Conventionally there is a problem that only theresponse characteristics corresponding to a feedback control band areexhibited in order to increase the data transfer speed of an opticaldisk apparatus. To resolve this problem, the semiconductor laser isparallelly driven by a plurality of driving elements, thereby obtainingresponse characteristics corresponding to an LD (laser diode) band.

The parallel driving also contributes to an increase in driving capacityof a semiconductor laser. When the semiconductor laser is driven by alarge amount of current, a transistor formed in an IC has to increase insize; however, there occurs a problem that the frequency characteristicsof the transistor is degraded. To resolve the problem the semiconductorlaser is parallelly driven by two driving transistors, thereby enhancingthe driving capacity without degrading the frequency characteristics.

FIG. 9 is a block diagram showing the principle of the parallel driving,Referring to FIG. 9, the output signals of a controllable amplifier 203and a buffer 206 are added to each other at an addition point 264 todrive an optical system (LD: laser diode) 205. The addition point 202,controllable amplifier 203, addition point 204, and optical system 205form a feedback loop, and its feedback band is about half the frequencyband of the single laser diode of the optical system 205 even by the useof wideband technology, since the feedback band is restricted by thefrequency band of all the laser diodes of the optical system 205.

If an input control signal is supplied through the buffer 206 andaddition point 204 to drive the laser diode of the optical system 205,the frequency characteristics of the system (laser diode) 205 can beexhibited sufficiently. However, when a wideband signal is input to theaddition point 204, the components more than those in the frequency bandof the optical system 205 causes a disturbance in the control loop, andan overshoot occurs in the response characteristics. To avoid theovershoot, an output signal of a compensator 207 having an inversecharacteristic of transmittance of the optical system 205, is input toan adder 201, and the output signal and the input signal of the controlloop are subjected to subtraction, thereby restricting the control band.

The foregoing parallel driving will be described with reference to FIGS.10 and 11.

In FIG. 10, line 302 a shows a closed-loop frequency characteristicobtained by the feedback control only, while line 302 b does aclosed-loop frequency characteristic obtained by the parallel driving.It is seen from FIG. 10 that the control band of the latter is two ormore times as wide as that of the former.

FIG. 11 is a graph representing pulse response characteristics ofoutputs of a laser diode of the optical system 205. In FIG. 11, curve303 a shows a pulse response characteristic obtained by the feedbackcontrol only, curve 303 b indicates a pulse response characteristicobtained when no transient compensation is performed in the paralleldriving, and curve 303 c shows a pulse response characteristic obtainedwhen transient compensation is performed in the parallel driving. It isapparent from curve 303 b that an over-shoot occurs and from curve 303 cthat an overshoot is greatly decreased. It is also apparent from curve303 c that the pulse rising time (Tr) and pulse falling time (Tf) areconsiderably improved, and the parallel driving increases the datatransfer speed further.

The above-described parallel driving is featured by driving an LD (laserdiode) in parallel by directly inputting a control signal to aconventional feedback system. The feedback system has a drawback ofcausing an overshoot since it is rendered in a noise-applied state. Toresolve this drawback, the control signal input to the feedback systemis corrected by a transient compensation means to avoid thenoise-applied state.

The equalizing compensation adopted in the third embodiment will now bedescribed in detail. The equalizing compensation aims at compensatingfor a distortion of frequency characteristics of a photo-detector (PD)incorporated in a semiconductor laser diode (LD). When a semiconductorlaser apparatus is applied to an information recording apparatus such asa laser beam printer, the gradation control of the intensity of laserbeams output from a high-speed semiconductor laser diode is required forhigh-precision printing (gradation printing) and high-speed printing.Since, however, the photodetector is used to detect a feedback output inorder to decrease in cost, its frequency characteristics we notgenerally flat because of a delay in scattering (which is caused by adelay in laser beam emitted outside a beam-receiving area), and suchfrequency characteristics have an influence upon the responsecharacteristics of the semiconductor laser diode. Avoiding the influencedue to the delay in scattering improves the response characteristics ofthe semiconductor laser diode. It is thus possible to greatly improve inboth printing quality and printing speed without causing a sag or thelike in the response characteristics of the semiconductor laser diode.

The equalizing compensation will be described with reference to FIG. 12.FIG. 12 is a block diagram of an equalizing compensation system forcompensating for undesired characteristics of the photodetector. Anaddition point 202, a control amplifier 203, and an optical system 205form a conventional wideband feedback loop. If, however, the frequencycharacteristics of a photodetector 212 of the optical system 205 aredeteriorated by a delay in scattering, their inverse characteristics arecaused in the output of a semiconductor laser diode 211 of the opticalsystem 205, as described above. To compensate for the inversecharacteristics, an equalizing filter 213 processes an input signal suchthat a signal input to a control loop has an inverse characteristic oftransfer function of the photodetector, and an output signal theequalizing filter 213 is input to an adder 210, thus subjecting thesignals input to both the adder 210 and control loop to subtraction.

The effects of the equalizing compensation will now be described withreference to FIGS. 13 and 14.

FIG. 13 shows simulation results of closed-loop frequencycharacteristics at an output point of the semiconductor laser diode 211.In FIG. 13, curve 305 a indicates a closed-loop frequency characteristicobtained only in the output of the wideband feedback loop and, in thiscase, the gain is increased in a wide range because of undesiredcharacteristics of the photodetector 212, while curve 305 b represents aclosed-loop frequency characteristic obtained when the equalizingcompensation is executed and this is a flattened, good characteristic.

FIG. 14 shows simulation results of pulse response characteristics. InFIG. 14, curve 306 a indicates a pulse response characteristic obtainedwhen no equalizing compensation is performed, and curve 306 b does apulse response characteristic (which can be approximated by the primaryhigh-pass filter characteristic) obtained when equalizing compensationis performed. When no equalizing compensation is performed, a sag occursbut it is completely compensated by the equalizing compensation. Theobjective of the wideband feedback loop is how the output of thesemiconductor laser diode 211 is equalized with the input controlsignal, and that of the equalizing compensation is to equalize theoutput point of the semiconductor laser diode 211 with the input controlsignal. The equalizing compensation can be achieved by processing theinput control signal using a filter having the saw transfercharacteristic as that of the semiconductor laser diode 211. If thecompensation function for the equalizing compensation is X(s), therequired condition is given by the following equation (1):

1−x(s)=1−P(s)  (1)

The compensation function X(s) can thus be obtained from the followingequation (2):

x(s)=1−P(s)  (2)

However, the compensation function need not be achieved faithfully as afilter, but can be approximated by a high-pass filter obtained bycutting off the pole representing the transfer function P(s) low-passfilter characteristics. For example, if the transfer function P(s) isgiven by the following equation (3).

P(s)=ω/(s+ω)  (3)

the compensation function X(s), which is expressed by the followingequation (4), shows a high-pass filter.

x(x)=s/(s+ω)  (4)

Conventionally, the frequency characteristics of the photodetectorincorporated in the semiconductor laser diode are not generally constantdue to a delay in scattering until the above cutoff is executed, and theoutput of the photodetector is decreased several decibels from 3 dB atfrequencies ranging from about 100 kHz to 20 MHz. If such aphotodetector is used for wideband feedback control, a problem ofcausing an inverse characteristic of an increase of several decibelsfrom 3 dB at frequencies ranging from about 100 kHz to 20 MHz occurs inthe feedback output frequency characteristics of the semiconductor laserdiode, as a feature of the feedback control, in order to compensate forthe decrease in the output. In this case, the pulse responsecharacteristics of output laser beams cause a sag and degrades thequality of printing. To resolve this problem, a control signal input tothe feedback system is corrected by the equalizing compensation toprevent a sag from occurring in the pulse response characteristics.

As has been described above, according to the present invention, sincethe first variable gain amplifier is provided at the first-stagecontrollable amplifying section, the dynamic range of the rear-stagecontrollable amplifying section with a higher gain is not degraded eventhough the gain of the first variable gain amplifier is varied. Inparticular, high-precision control less vulnerable to a variance inthreshold current can be achieved.

Furthermore, since the second variable gain amplifier is independent ofthe feedback control loop, the phase compensation of the feedback can beadjusted independently. Since the gain of the second variable gainamplifier is variable, the constant of the compensation means need notbe altered, thus lowering the costs for parts and adjustment.

When the semiconductor laser apparatus of the present invention isapplied to an information recording/reproduction apparatus, it isresistant to a variance in several factors and its manufacturing yieldis improved.

When the semiconductor laser apparatus is applied to an image recordingapparatus, high-speed, high-quality characteristics can be achieved byvirtue of high light turn-off ratio, high-precision light quantity, andhigh-speed, stable pulse characteristics.

When a semiconductor laser apparatus wherein the driving responsecharacteristics of a semiconductor laser diode are improved by theparallel driving and transient compensation, is applied to an opticaldisk apparatus, the speed of data transfer can be increased.

When a semiconductor laser apparatus wherein the distortion of frequencycharacteristics of a photodetector of a semiconductor laser diode iscorrected by the equalizing compensation, is applied to a laser printer,both printing quality and printing speed can be greatly increased,without causing any sag in the pulse response characteristics of outputlaser beams.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, and representative devices shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A semiconductor laser apparatus comprising: asemiconductor laser; a photodetector for detecting an output beam ofsaid semiconductors laser; a driving unit having a plurality of drivingelements connected in parallel, for driving said semiconductor laser; acontrollable amplifying unit for receiving an externally suppliedcontrol signal and a monitor signal of said photodetector and supplyinga drive signal determined by said control signal and said monitor signalof said photodetector to said driving unit, thereby feedback-controllingthe output beam of the semiconductor laser; and a compensation unit foractively controlling a phase of a feedback control loop constituted bysaid photodetector, said driving unit and said controllable amplifyingunit in accordance with temporal and electrical behaviors of thefeedback control loop.
 2. The semiconductor laser apparatus according toclaim 1, wherein said compensation unit comprises a variable gainamplifier and a CR circuit including a capacitance and a resistor, thecontrol signal and the output beam of said photodetector are deliveredto input terminals of said variable gain amplifier, an output terminalof said variable gain amplifier is connected to one terminal of said CRcircuit, and the other terminal of the CR circuit is connected to aninput of said controllable amplifying unit.
 3. The semiconductor laserapparatus according to claim 2, wherein said variable gain amplifier hasflat frequency characteristics up to a frequency band of at least 1.5times the control band of said variable gain amplifier.
 4. Thesemiconductor laser apparatus according to claim 1, wherein saidcontrollable amplifying unit includes at least a variable gaindifferential amplifier functioning as a first-stage amplifier, and arear-stage amplifier having a higher gain than said first-stageamplifier.
 5. The semiconductor laser apparatus according to claim 4,wherein said controllable amplifying unit includes a high-frequencyaddition unit provided between said first-stage amplifier and saidrear-stage amplifier, said high-frequency addition unit adding ahigh-frequency signal to an output of said first-stage amplifier.
 6. Thesemiconductor laser apparatus according to claim 4, wherein saidcontrollable amplifying unit includes a D/A conversion unit forconverting an externally supplied digital signal to an analog controlsignal and supplying the analog control signal to said first-stageamplifier in order to control the output beam in accordance with theexternally supplied digital signal.
 7. The semiconductor laser apparatusaccording to claim 1, further comprising a transient compensation unitfor preventing an overshoot from occurring in the feedback control loop.8. The semiconductor laser apparatus according to claim 1, furthercomprising an equalizing compensation unit for compensating for acontrol signal input to the feedback control loop to prevent a sag fromoccurring in pulse response characteristics of the output beam of saidsemiconductor laser.
 9. The semiconductor laser apparatus according toclaim 1, wherein said semiconductor laser, said photodetector, saiddriving unit, said controllable amplifying unit and said compensationunit are formed on one chip.
 10. The semiconductor laser apparatusaccording to claim 1, wherein said semiconductor laser, saidphotodetector, said driving unit, said controllable amplifying unit andsaid compensation unit are formed on one chip, and said apparatus isapplied to an information recording apparatus.
 11. The semiconductorlaser apparatus according to claim 1, wherein said semiconductor laser,said photodetector, said driving unit, said controllable amplifying unitand said compensation unit are formed on one chip, and said apparatus isapplied to an information recording/reproduction apparatus.
 12. Asemiconductor laser control device for supplying a high-frequencysuperimposed drive current to a semiconductor laser, comprising: acontrol circuit which receives a control signal and processes thecontrol signal to obtain a signal output corresponding to an amount ofemission light from semiconductor laser; a high-frequency signalgenerating circuit which generates a high-frequency signal for reducingnoise caused by light returning to the semiconductor laser; and anaddition unit which adds the high-frequency signal to the signal outputfrom the control circuit, wherein the control circuit, thehigh-frequency signal generating circuit, and the addition unit areintegrally fabricated on one chip.
 13. A semiconductor laser controldevice according to claim 12, wherein the control circuit includes anamplifier having a gain control terminal.
 14. A semiconductor lasercontrol device according to claim 12, wherein the control circuitreceives a detection signal, other than the control signal, obtained bydetecting an output beam of the semiconductor laser.
 15. A semiconductorlaser control device according to claim 12, further comprising a drivertransistor which outputs the high-frequency superimposed drive currentto the semiconductor laser.
 16. A semiconductor laser control device forsupplying a high-frequency superimposed drive current to a semiconductorlaser, comprising: a control circuit which receives a control signal andprocesses the control signal; a high-frequency signal generating circuitwhich generates a high-frequency signal for reducing noise caused bylight returning to the semiconductor laser; a terminal disposed tocouple the high-frequency signal generating circuit with an externalresistor; and an addition unit which adds the high-frequency signal toan output signal from the control circuit, wherein the control circuit,the high-frequency signal generating circuit, the terminal, and theaddition unit are integrally fabricated on one chip.
 17. A semiconductorlaser control device according to claim 16, wherein the control circuitincludes an amplifier having a gain control terminal.
 18. Asemiconductor laser control device according to claim 16, wherein thecontrol circuit outputs a signal for adjusting an amount of emissionlight from the semiconductor laser.
 19. A semiconductor laser controldevice according to claim 16, wherein the control circuit receives adetection signal, other than the control signal, obtained by detectingan output beam of the semiconductor laser.
 20. A semiconductor lasercontrol device according to claim 16, further comprising a drivertransistor which outputs the high-frequency superimposed drive currentto the semiconductor laser.
 21. A semiconductor laser control deviceincluding an auto-power-control circuit for negatively feeding back adetection signal obtained by detecting an output beam of a semiconductorlaser to control an amount of emission light from the semiconductorlaser, wherein the improvement comprises: a high-frequency signalgenerating circuit integrated with the auto-power control circuit withinone chip, the high-frequency signal generating circuit generating ahigh-frequency signal for reducing noise caused by light returning tothe semiconductor laser.
 22. A semiconductor laser control deviceaccording to claim 21, wherein the auto-power-control includes anamplifier having an inverted input terminal supplied with the detectionsignal.
 23. A semiconductor laser control device according to claim 21,further comprising a driver transistor which outputs a high-frequencysuperimposed drive current to the semiconductor laser.